阅读程序2解析

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/**************************************************************** 
 * Description: 2021_J_finish_program_2
 * Date: 2023-08-31 10:02:17
 * LastEditTime: 2023-08-31 21:06:23
****************************************************************/
#include <iostream>
    #include <string>
    using namespace std;

    char base[64];
    char table[256];
//初始化base和table数组 
    void init(){
        for (int i = 0; i < 26; i++) base[i] = 'A' + i; //base数组0～25存A～Z
        for (int i = 0; i < 26; i++) base[26 + i] = 'a' + i; //26~51存a~z
        for (int i = 0; i < 10; i++) base[52 + i] = '0' + i; //52~61 存0～9
        base[62] = '+', base[63] = '/';  // 62存+，63存/

         for (int i = 0; i < 256; i++) table[i] = 0xff; //char类型，用16进制初始化，11111111=-1  
         for (int i = 0; i < 64; i++) table[base[i]] = i; //对table按base数组的值做下标进行赋值
        table['='] = 0;
    }

    string decode(string str){
        string ret;
        int i=0;
           
         for (i = 0; i < str.size(); i += 4) { //循环按每4个
         //    ret+=table[str[i]];
           ret += table[str[i]] <<2  | table[str[i + 1]] >> 4; //生成新字符给ret字符串
        
             if (str[i + 2] != '='){ //第三个字符如果不等于=
                 ret += (table[str[i + 1]] & 0x0f) << 4 | table[str[i + 2]] >> 2;
                 if (str[i + 3] != '=')  //第四个字符如果不等于=
                  ret += table[str[i + 2]] << 6 | table[str[i + 3]];
        }
        }
        return ret;
    }

   int main(){
        init();
        for (int i = 0; i <64; i++){
             cout<<base[i]<<' ';
         }
         cout<<endl;
        for (int  i = 0; i < 256; i++){
            cout<<(int)table[i]<<' ';
        }
        

       cout << ((int)table[0])<< endl;
      
       string str;
        cin >> str;
     //   decode(str);
      cout<<decode(str)<< endl;
        return 0;
    }